1. Field of Invention
The present invention relates to a surface light source device of side light type and a liquid crystal display employing the surface light source device in its backlighting arrangement. For instance, the present invention relates to a liquid crystal display provided with a liquid crystal display panel employing TN liquid crystal.
2. Related Art
Surface light source devices of side light type have been conventionally employed, for instance, to illuminate a liquid crystal display panel from its back. This arrangement is suitable for making the overall structure thin.
In general, the light source device of side light type comprises a rod-shaped light source such as a cold cathode tube as a primary light source, which is arranged beside a guide plate (i.e. plate-shaped light guide). The primary light source emits illumination light which is introduced into the guide plate through a side end face (i.e. incidence face) thereof. Such introduced light propagates inside of the guide plate while outputting emission from a major face (i.e. emission face) toward a liquid crystal display panel.
Known types of guide plates employable in light source devices of side light type are, one having a substantially uniform thickness and another having a tendency to reduce thickness according to distance from the primary light source. The latter provides emission, in general, more effectively in comparison with the former. Light scattering-and-guiding material or transparent resin is employed as a material of these guide plates.
A guide plate made of light scattering-and-guiding material is called a xe2x80x9cscattering guide platexe2x80x9d. A scattering guide plate is composed of a matrix, such as PMMA (polymethlyl methacrylate), and a great number of light-permeable fine particles which are dispersed uniformly therein. The fine particles are different from the matrix in refractive index.
FIG. 11 is an exploded perspective view illustrating a conventional light source device of side light type which employs a guide plate of the latter sort. FIG. 12 is a cross section along line 12xe2x80x9412 in FIG. 11.
Referring to FIG. 11 and FIG. 12, a light source device of side light type 1 comprises a guide plate 2, a primary light source 3, a reflection sheet 4 and a prism sheet 6 which is functioning as a light control member. The reflection sheet 4, guide plate 2 and prism sheet 6 are laminatedly arranged. The primary light source 3 is disposed beside the guide plate 2.
The guide plate 2 is made of a scattering-and-guiding material with a wedge-shaped cross section, being called a scattering guide plate. The scattering-and-guiding material is composed of a matrix, such as PMMA (polymethyl methacrylate), and a great number of light-permeable fine particles which are dispersed uniformly therein. The fine particles are different from the matrix in refractive index.
The primary light source 3 includes a cold cathode tube (fluorescent lamp) 7 backed by a reflector 8 which is generally semi-circular in cross section. Illumination light is supplied to a side end face, an incidence face 2A, of the scattering guide plate 2 through an opening of the reflector 8. The reflection sheet 4 is a sheet-like member with regular reflectivity such as metal foil or with irregular reflectivity such as white PET film.
Illumination light L is introduced into the guide plate 2 through the incidence face 2A and propagates toward a distal end while repeating reflections at two major faces (back face 2B and emission face 2C). Illumination light is therewith subject to scattering effect of the fine particles within the guide plate 2. If the reflection sheet 4 is an irregular reflection member, there will be an added irregular reflection effect.
Repeated reflections by the inclined back face 2B will give the illumination light reducing incidence angles with respect to the emission face 2C. Such reduction in incidence angle brings increased components which are angularly smaller than the critical angle with respect to the emission face 2C, thereby promoting emission from the emission face. This prevents emission intensity from being insufficient in an area distant from the primary light source 3.
The emission face 2C outputs illumination light which assumes scattering light because it has undergone scattering effect of the fine particles within the guide plate 2 and, in some cases, further has undergone irregular reflection effect of the reflection sheet 4.
However, illumination light outputted from the emission face 2C has a principal propagation direction inclined toward the distal end with respect to the frontal direction in a plane perpendicular to the incidence face 2A (i.e. inclined toward a direction as distancing the incidence face 2A). That is, the output light of the scattering guide plate 2 has directivity. This property is called xe2x80x9cemission directivityxe2x80x9d.
The prism sheet 6 disposed along the emission face 2C is a light-permeable sheet made of, for instance, polycarbonate. The prism sheet 6 is provided with a prismatic surface including a great number of parallel prismatic rows. In the illustrated example, the prism sheet 6 is arranged in an orientation such that the prismatic surface is directed toward the guide plate 2 and the prismatic rows run generally parallel (within an angular range about 0 to 15 degrees) to the incidence face 2A.
Each prismatic row is a fine projection row such as one having a triangular cross section. Slopes of the projections correct obliquely emitted light toward the frontal direction in a plane perpendicular to the incidence face 2A.
A double-surfaces prism sheet with prismatic surfaces on both faces may be employed. In the case, prismatic rows on its outer face run generally at right angles with ones on its inner face. The prismatic rows on the outer face correct light angularly toward the frontal direction in a plane parallel to the incidence face 2A.
FIG. 13 is an exploded perspective view illustrating another conventional light source device of side light type. FIG. 14 is a cross section along line 14xe2x80x9414 in FIG. 13. In this example, a transparent guide plate is employed.
Referring to FIG. 13 and FIG. 14, a light source device of side light type 10 comprises a guide plate 12, a primary light source 3, a reflection sheet 4, a light diffusion sheet 13 and prism sheets 14, 15. The reflection sheet 4, guide plate 12, light diffusion sheet 13 and prism sheets 14, 15 are laminatedly arranged.
The guide plate 12 is made of a molded body of transparent acrylic resin, having a wedge-shaped cross section. The guide plate 12 has major faces to provide a back face 12B and an emission face 12C. A great number of dot-like light diffusion elements form a pattern on the back face 12B, thereby providing a light diffusible surface. In order to achieve uniform brightness, the dot pattern is formed so that diffusing power rises according to distance from an incidence face 12A. For instance, a covering rate of the dot pattern is adjusted so as to increase according to distance from the incidence face 12A.
The primary light source 3 and the reflector 4 are the same as those shown in FIGS. 11 and 12. Illumination light L emitted from the primary light source 3 is introduced into the guide plate 12 through the incidence face 12A. Such introduced light L propagates toward a distal end while repeating reflections at the back face 12B, along which the reflector is disposed, and the emission face 2C. On the way, illumination light L is subject to scattering effect of the back face 12B which is provided with diffusibility. If the employed reflection sheet 4 has irregular reflectivity, irregular reflection will be effected.
Repeated reflections by the inclined back face 12B will give the illumination light reducing incidence angles with respect to the emission face 12C. Such reduction in incidence angle brings increased components which are angularly smaller than the critical angle with respect to the emission face 2C, thereby promoting emission from the emission face. This prevents emission intensity from being insufficient in an area distant from the primary light source 3. Illumination light outputted from the emission face 12C has scattering property as well as directivity similar to that in the case shown in FIGS. 11 and 12. The prism sheets 5, 6 are arranged to correct emission directivity of the guide plate 12.
The light diffusion sheet 13 diffuses weakly illumination light emitted from the guide plate 12, thereby preventing the diffusible surface on the back face 12B from being seen through. If such prevention is not supplied the diffusible surface will be observed from above the emission face 12C, causing reduction in illumination light quality.
The prism sheets 5, 6 are light-permeable sheets made of, for instance, polycarbonate. Prismatic surfaces including a great number of projections are formed on faces which are opposite with the guide plate 12 (i.e. on outer faces).
Each projection consists of a pair of slopes forming an isosceles triangular cross section. The prism sheet 14 is orientated so that projections on its prismatic surface run generally in line (within an angular range from 0 to about 15 degrees) with the incidence face 12A. On the other hand, the prism sheet 15 is orientated so that projections on its prismatic surface run generally at right angles (within an angular range of 90xc2x1about 15 degrees) with respect to the incidence face 12A.
Main propagation direction of the light outputted from the emission face 12C is corrected by the prism sheet 14 toward the frontal direction in a plane perpendicular to the incidence face 12A. And then the prism sheet 15 gathers angularly the illumination light toward the frontal direction in a plane parallel to the incidence face 12A. A double-surfaces prism sheet with prismatic surfaces on both faces may be employed. The surface light source device as described in this example is also capable of outputting generally uniform emission toward the frontal direction as well as the previously described prior art device (as shown in FIGS. 11, 12).
If a surface light source device as described above is applied to backlighting arrangement for LCD, a liquid crystal display panel is disposed outside of the prism sheet 6 (FIG. 11) or prism sheet 15 (FIG. 13). Backlighting of the liquid crystal display panel is effected through a polarizer arranged on a back face of the liquid crystal cell. As broadly known, the polarizer transmits selectively a polarization component which accords directionally with its polarization plane. Such a polarization plane of a polarizer as employed in a liquid crystal display panel is called xe2x80x9ctransmission polarization plane of liquid crystal display panelxe2x80x9d.
Illumination light from a surface light source of above-described sorts contain polarization components of all directions. Every polarization component which does not match the transmission polarization plane is almost absorbed and this leads to useless consumption. As a result, a bright display with low electric power is hardly realized.
To avoid this, it has been proposed to put a polarization separating element before the polarizer. The polarization separating element is an element, for example, consisting of a multilayer film made of optical anisotropy materials which have different refractive indexes for an ordinary ray and an extraordinary ray. It has a function such that a polarization component directionally according to its polarization plane is transmitted while polarization component directionally different from its a polarization plane is almost reflected. Such polarization plane set at the polarization separating element is called xe2x80x9ctransmission polarization plane of polarization separating elementxe2x80x9d. And a plane perpendicular to this is called xe2x80x9creflection polarization planexe2x80x9d.
Thus a reflected component provides xe2x80x9creturning lightxe2x80x9d which travels toward the guide plate. Such returning light is converted into light like natural light via return to the guide plate, re-emission therefrom and other various routes, being subject to recyclic use. Such recyclic light impinges the polarization separating element again and a remarkable portion (for instance, more than about a half) of this incident light transmits through the polarization separating element. Accordingly, if the polarization separating element and the liquid crystal display panel are orientated so that their transmission polarization planes accord with each other, backlighting efficiency is improved.
This will be understood by referring to FIGS. 15 and 16. FIG. 15 is a graph illustrating directional luminance characteristics (measurement result) of an arrangement in which a liquid crystal display panel (polarizer) is backed by the surface light source device 10 as shown in FIG. 13 (without a polarization separating element). In the graph, luminance indication is normalized so that luminance of the vertical direction with respect to the emission face 12C is indicated by the standard value xe2x80x9c1xe2x80x9d. A white PET sheet was employed as the reflection sheet 4.
On the other hand, FIG. 16 is a graph illustrating directional luminance characteristics (measurement result) of an arrangement in which a polarization separating element is interposed between the liquid crystal display panel (polarizer) and the surface light source device 10. Luminance curves are plotted according to the normalization adopted in FIG. 15. Comparing FIG. 16 with FIG. 15, luminance of vertical direction with respect to the emission face 12C is increased by a factor of 1.247, which is due to the interposed polarization separating element.
A similar comparison was performed also for cases where the surface light source device 1 shown in FIG. 11 was employed for backlighting. Results are shown in FIGS. 17 and 18.
FIG. 17 is a graph illustrating directional luminance characteristics (measurement result) of an arrangement in which a liquid crystal display panel (polarizer) is backed by the surface light source device 1 (without a polarization separating element). On the other hand, FIG. 18 is a graph illustrating directional luminance characteristics (measurement result) of an arrangement in which a polarization separating element is interposed between the liquid crystal display panel (polarizer) and the surface light source device 1.
Luminance curves in these (FIGS. 17 and 18) are plotted according to the normalization adopted in FIG. 15. Comparing FIG. 17 with FIG. 15, luminance of the frontal direction is reduced by a factor of 0.705. Such reduction is supposedly due to absence of prism sheet to gather light in a plane parallel to the incidence face 2A. Nevertheless, as understood from FIG. 18, the interposed polarization separating element increases luminance of the frontal direction to 1.001 times that of FIG. 15.
From the above-demonstrated results, there is no doubt that employment of a polarization separating provides improvement in luminance of an LCD (i.e. improvement in displaying luminance). However, room for further improvement in luminance yet remains for the following reasons.
As described above, luminance improvement effect of the polarization separating element is dependent on a phenomenon that components discording to the transmission polarization plane are after reflection, directed to recycle use via various routes. Roughly estimated, if this recycle process is perfect (i.e. involving no loss) total emission from the liquid crystal display panel is expected to be doubled due to the polarization separating element.
To see this, integration of luminance depending on directions was calculated for the cases of FIG. 15 and FIG. 16, respectively. The calculation told that luminance improvement due to the polarization separating element was a factor of 1.577. According to similar calculations for comparing the case of FIG. 17 and FIG. 18, luminance improvement due to the polarization separating element was a factor of 1.578.
From such results, it is understood that room for further increase in recycle efficiency of components reflected by the polarization separating element (i.e. returning light) remains yet. Such further increases in recycle efficiency would provide improvement in displaying brightness of the LCD.
The present invention is proposed under the above-described background. An object of the present invention is to provide a novel surface light source device of side light type which is capable of increasing emission from a liquid crystal display panel strikingly compared with prior art surface light source devices provided with an arrangement including a polarization separating element.
Another object of the present invention is to provide a liquid crystal display which is improved in displaying brightness due to said novel surface light source device of side light type. The present invention is applied to a surface light source device of side light type which comprises a guide plate having two major faces to provide an emission face and a back face, a primary light source for supplying illumination light to a side end face of the guide plate and a polarization separating element arranged in a light path of emission from the emission face.
The polarization separating element has a function to transmit polarization components directionally corresponding to a transmission polarization plane while reflecting polarization components directionally corresponding to a reflection polarization plane which is perpendicular to the transmission polarization plane. The back face of the guide plate is provided with a great number of projection rows which run parallel with each other and in a first direction.
According to a feature of the present invention, the first direction is neither corresponding to the transmission polarization plane nor vertical with respect to the transmission polarization plane. In a preferable embodiment, the first direction gives an approximate bisect of an angle between the transmission polarization plane and the reflection polarization plane. The first direction may be generally vertical with respect to the side end face.
A light control member may be interposed between the emission face and the polarization separating element. In this case, the light control member is provided with a great number of projection rows which run parallel with each other and in a second direction vertical with respect to the first direction. The polarization separating element and the light control member may be unified into one member.
Individual projection rows on the back face of the guide plate may include a pair of slopes. Each pair of slopes preferably give an angle within a range from 50 to 130 degrees. Individual projection rows of the light control member may also include a pair of slopes. Each pair of slopes preferably give an angle within a range from 30 to 70 degrees. A reflection surface is arranged along the back face of the guide plate.
To apply a surface light source device as above-described to a liquid crystal display, a liquid crystal display panel may be backed by the light source device as to be supplied with illumination light through a polarizer. In this case, the polarizer is set on an incidence side of the liquid crystal display panel so that the polarizer and the polarization separating element have transmission polarization planes, respectively, which correspond directionally to each other.
Features of the present invention enable reflection at the polarization separating element to provide returning light which is subject to inside reflection by the projection rows formed on the back face of the guide plate, thereby causing the guide plate to emit afresh recycling light. This recycling light, is rich with the polarization component directionally corresponding to the transmission polarization plane of the polarization separating element. Accordingly, the illumination light supply to the liquid crystal display panel will involve low loss, leading to improvement in displaying brightness.
More details and features of the present invention will be understood by the following description with references to the drawings in which: